1. Field of the Invention
This invention relates to composite superconductors, and more particularly to composites comprising high field superconductors of the A-15 crystal structure type.
2. Description of Prior Art
Superconducting materials are those materials which exhibit zero resistance when the temperature of the material is lowered below some critical temperature level. At this critical temperature the material undergoes a transition from the normal to the superconducting state. The temperature at which this transition takes place is exceedingly important, that is, a high critical temperature is very desirable.
Another important superconducting property is that the material should have a high critical current. That is, the maximum current carried before a measurable voltage first appears in the superconducting material must be high.
Therefore, materials which exhibit a high critical temperature, a high critical field and a high critical current are preferred superconductors. Such materials are intermetallic compounds of the A-15 crystal structure including Nb.sub.3 Sn; Nb.sub.3 Ga; V.sub.3 Ga; Nb.sub.3 Al and Nb.sub.3 Ge. The principal difficulty with these materials that has inhibited their acceptance is their inherent brittleness, that is, they easily fracture when stressed. A superconductor must be capable of being wound into a solenoid without subsequent deterioration in its properties, such as would occur if the superconductive compound would fracture during winding.
Composite superconductors of the A-15 type have been made of niobium-tin in the form of a tape. A thin deposit of Nb.sub.3 Sn is placed onto a supporting metallic substrate so that the composite structure can be stressed without damage to the superconductor. This can be accomplished by vapor deposition and by diffusion.
In vapor deposition a mixture of gaseous niobium and tin chlorides are deposited on a hot stainless steel strip. By proportioning of the chlorides the deposit will consist of Nb.sub.3 Sn.
In the diffusion process an initial layer of tin of a specific predetermined thickness is formed on a niobium strip and the resulting laminate is treated at 1000.degree.C to form Nb.sub.3 Sn by reaction between the tin and niobium.
The Nb.sub.3 Sn composite is then stabilized by laminating it between layers of OFHC copper thereby producing a laminated conductor having superconductive material surrounded by normal material. The high thermal conductivity and high electrical conductivity of the copper provides a protective current shunt in the event that the superconductor should momentarily transform from the superconducting to the normal state.
Aside from the inherent brittleness of tapes comprising an intermetallic compound, the geometry of the product was such that compact, stabilized coils could not be wound. Furthermore, intrinsically stable conductors are generally twisted about a central axis with a pitch of a few inches. This can easily be accomplished in composite filaments of 0.01 - 0.05 inches in diameter, however it is far more difficult to impart such a pitch to a tape of say 0.5 inches wide by 0.01 inches thick.
Recently multifilament A-15 type superconducting wire has been introduced as an alternate to the A-15 tape hereinbefore described. As set forth in British Pat. Spec. No. 1,280,583 a method is disclosed for making multifilament composite superconductors of the A-15 type.
The disclosed method overcomes one of the principal disadvantages of superconductive tapes consisting of intermetallic compounds, namely brittleness. In this method rods of pure niobium are inserted into a solid copper-tin alloy matrix or into a particle mass of copper and tin powders. The assembly is then mechanically worked to fine wire. The initial rods are now filamentary strands that are converted into a superconductive compound by heating the wire at 773.9.degree.C (1425.degree.F) for about 96 hours. This heat treatment permits niobium and tin to react and form Nb.sub.3 Sn on the periphery of the filaments.
Although this method produces satisfactory multifilament Nb.sub.3 Sn we have found that the overall superconducting properties to be less than satisfactory. As we have previously indicated a stabilized superconductor must have a quantity of high purity normal material interposed in the conductor to act as a shunt in the event the conductor transforms from the superconducting to the normal states. High purity copper, silver and aluminum can be used. These materials exhibit high electrical and thermal conductivities. It is well known in the art that conductivity of the normal material can be seriously degraded by the slightest amount of contamination.
To stabilize a multifilament superconductor as taught by the aforementioned British Specification, high purity (OFHC) copper rods could be inserted into the copper-tin matrix or a copper sheath could be used as a jacket for the assembly. The conductivity of the copper would not be affected during mechanical working down to fine wire. However, once the high temperature heat treatment starts tin will not only diffuse into the niobium filaments to form Nb.sub.3 Sn; it will also diffuse into the high purity copper thereby destroying the thermal and electrical conductivity of this material. The final product will not be a stabilized superconductor.